This invention relates to an improvement in phase zone plate optics that embraces contact and intraocular lenses. A "phase zone plate", as employed herein and in the claims, is a unitary optical region of a lens utilizing the combination of a zone plate and optical facets (such as in the form of echelettes) in the zones of the zone plate, and the combined facets in the zones diffract light to produce a specific wavefront which results in a specific intensity distribution of light at a variety of orders (e.g., 0.sup.th, 1.sup.st, etc.) of the zone plate. The orders constitute the foci of the zone plate. In a restrictive sense and also in the most utilitarian sense, the phase zone plate is designed for general lens applications where the distribution of light at effective intensities is dependent upon zone spacing for yellow light. Yellow light, as employed herein, is that portion of the visible spectrum at 530-570 nanometers.
This invention relates inter alias to contact lenses. Contact lenses are classical vergency type lenses. They possess a concave corneal bowl (the posterior surface) that allows fitting to the eye and the outer surface (the anterior surface) is smooth and shaped to allow the eyelid to slide over the eye and to provide proper verging of light (taking the lens material's refractive index into consideration) to a focal point accommodating to the eye. The majority of the commercial contact lenses are shaped such that the lenses are thinnest about the optical axis and the thickness of the lenses gradually increases along a sloped radial length extending from the optical axis toward the lens perimeter. Owing to this variation in thickness, light passing through the optical axis has to pass through less lens material. Because light travels faster in air than it does through the lens, the light passing through the thicker portions of the lens will be shifted, hence be retarded in time..sup.1 Consequently, the shape of the lens is selected to accommodate this progressive retardation of the light so that the lightwaves emanating from the posterior surface are in synchronization in reaching a desired focal point. FNT 1. See Fincham, et al., Optics, Published by Butterworths, London, 9.sup.th edition, 1980, 1981, pages 72-75.
This invention concerns contact lenses utilizing phase zone plate optics such as phase zone plate bifocals and "tuned" Fresnel lenses making use of concentric annular zones. Such lenses generally follow the designs described, for example, by Allen L. Cohen in U.S. Pat. Nos. 4,210,391; 4,338,005; and 4,340,283 ("Cohen patents"). A Cohen lens design provides that the radii "r.sub.k " of the annular and concentric zones are substantially proportional to .sqroot.k and that the zones are cut so as to direct light to more than one focal point.
The Cohen lens design with phase zone plate optics allows bifocal lens constructions which are exceptionally thin. Contact lenses may be designed with phase zone plate optics in order to achieve a bifocal or other multifocal effects. The specific chromatic properties of a phase zone plate may be incorporated in the design of a contact lens including a contact lens having multifocal properties. All phase zone plate optical elements which are designated bifocals are possessed inherently with the ability to focus light to more than two focal points. They are designated bifocals because the intensity levels of the light to any two orders, e.g., the 0.sup.th and 1.sup.st order focal points are adequate for bifocal applications. In that sense, every bifocal distributes light to a third, and possibly more, focus. The judgment of whether a lens is a bifocal or trifocal is not based on any strict rule. If the wearer of the lens does not find objectionable the presence of the third or more focuses, then the lens is probably adequate as a bifocal. .sup.2 FNT 2. See Klein and Ho, SPIE, August 1986, Table 2 and the comments about Table 2.
Other references mentioning or suggesting phase zone plate optics in regards to contact lenses are G. Forst, "Investigations into the Usability of Circular Grating as Vision Aids" Der Augenoptiker, 1966 (12), pages 9-19; Ziegler, "Fabrication or Correction of Optical Lenses," as modified by Cohen, see column 4, lines 27-36 of Cohen, U.S. Pat. No. 4,338,005, and column 5, line 63 to column 6, line 68, of Cohen, U.S. Pat. No. 4,210,391; Freeman, U.S. Pat. No. 4,637,697; and Freeman, U.S. Pat. No. 4,642,112 (to the extent that holography embraces phase zone plate optics).
A full-period zone, for purposes of this invention, is defined as the smallest repetitive sequence of facets within a phase zone plate which are spaced substantially proportional to .sqroot.k. Such spacing is characterized by the formula: ##EQU2## where f represents the 1.sup.st order focal length. A half-period zone, for the purposes of this invention, is characterized by the formula: ##EQU3## where f represents the 1.sup.st order focal length.
The non-refractive step wall or riser to the plateau of the step is cylindrical or nearly cylindrical in the planar direction of the optical axis of the lens, and thereby occupies a small fraction of the lens phase zone plate surface area.